1. Field of the Invention
The present invention relates to a burner used when fabricating an optical fiber perform by hydrolyzing glass raw material gas in flame to generate glass particles and depositing the glass particles on a rotating start rod.
2. Description of the Related Art
Up until now, various methods have been proposed for fabricating optical fiber preforms. Among these methods, an Outside Vapor Phase Deposition (OVD) Method), in which glass particles generated in burner flame are adhered and deposited on a rotating start rod while relatively reciprocating the burner and the start rod to synthesize a porous preform and the preform is dehydrated and sintered in an electric furnace, is widely used because the method can make it relatively easy to fabricate an optical fiber perform having a desired refractive index profile and can mass-produce a large-diameter optical fiber perform.
FIG. 1 schematically shows an example of an apparatus for fabricating an optical fiber preform. In FIG. 1, a start rod, on which glass particles are to be deposited, is constituted by welding a dummy rod 2 at the both ends of a core rod 1. The start rod is held by chucking and rotating mechanisms 4 at both ends thereof. Each of the chucking and rotating mechanisms 4 is mounted on a supporting member 7 and rotates the start rod about the axis thereof. A burner 3 sprays vapor of an optical fiber raw material such as SiCl4 and combustion gas (hydrogen gas and oxygen gas) on the start rod, which is rotated about the axis thereof, while being reciprocated in longitudinal directions of the start rod by a burner moving mechanism 6. As a result, glass particles generated by the hydrolysis in oxyhydrogen flame are deposited on the start rod so that a porous optical fiber preform is formed. Meanwhile, the reference numeral 5 indicates an exhaust hood for the vapor and the combustion gas.
For synthesizing glass particles and depositing the glass particles on a start rod, a burner having a coaxial multiple tube structure has been conventionally used. However, in the burner having such structure, sufficient synthetic efficiency of the glass particles can not be obtained, because a glass raw material gas, a burnable gas and a combustion assisting gas are not sufficiently mixed with each other. As the result, the yield has not been improved and a high speed synthesis of a porous optical fiber preform has been difficult.
In order to solve the problem, Japanese Patent No. 1773359 (correspondent to European Patent No. 0237183 and U.S. Pat. No. 4,810,189) proposed a multi-nozzle type burner, in which small diameter combustion assisting gas nozzles (hereinafter, abbreviated as small diameter nozzle) are arranged in a burnable gas port so as to surround the central raw material gas port.
For the burner of this type, some methods for further improving the deposition efficiency have been proposed. For example, Japanese Patent Application Laying-Open Nos. 2003-206154, 2004-331440 and 2006-182624 (corresponding to U.S. Patent Application Laying-Open No. 2006137404), and Japanese Patent No. 3744350 disclose the arrangements of the small diameter nozzles; Japanese Patent Application Laying-Open No. H05-323130 (1993), Japanese Patent No. 3543537 and Japanese Patent Application Laying-Open No. 2003-226544 disclose mechanisms for optimizing focal length of the small diameter nozzle; and Japanese Patent No. 3591330, Japanese Patent Application Laying-Open Nos. 2003-165737 and 2003-212555, and Japanese Patent No. 3653902 disclose mechanisms for optimizing gas flow rate and gas linear velocity.
Conventionally, there has been a problem that glass particles being floated by the disturbance of gas flow easily adhere to the front end of eject ports located outside a glass raw material gas port to close the burner front end when the adhesion proceeds. Particularly, in a burner provided with the small diameter nozzles, a plurality of small diameter nozzles are arranged in the portion near the central glass raw material gas port, and thus the burner is constructed such that glass particles adhere easily to the front end of the small diameter nozzle.
In order to overcome the problem, the front end of the central glass raw material gas port and the front end of the small diameter nozzles are set apart from each other in a radial direction so as to avoid the adhesion of glass particles. However, the diameter of the burnable gas port covering the small diameter nozzles must be made large, which leads to the enlargement of a flow path area to generate such new problem as the increase in a necessary flow rate of the burnable gas. In addition, the enlargement of a burner in size makes the setting of the burner in a deposition apparatus difficult.